dropbearkey: add page (#9945)

* dropbearkey: add page

* Apply suggestions from code review

Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>

---------

Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>

pages/linux/dropbearkey.md

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+# dropbearkey
+
+> Generate SSH keys in Dropbear format.
+> More information: <https://manned.org/dropbearkey.1>.
+
+- Generate an SSH key of [t]ype ed25519 and write it to key [f]ile:
+
+`dropbearkey -t {{ed25519}} -f {{path/to/key_file}}`
+
+- Generate an SSH key of [t]ype ecdsa and write it to key [f]ile:
+
+`dropbearkey -t {{ecdsa}} -f {{path/to/key_file}}`
+
+- Generate an SSH key of [t]ype RSA with 4096-bit key [s]ize and write it to key [f]ile:
+
+`dropbearkey -t {{rsa}} -s {{4096}} -f {{path/to/key_file}}`
+
+- Print the private key fingerprint and public key in key [f]ile:
+
+`dropbearkey -y -f {{path/to/key_file}}`